The present invention relates to an information storage system having the function of copying or compressing and storing updated data as secondary data among one or more control units.
International application published under the patent cooperation treaty WO 94/25919 relates to duplication of data which is sent/received among control units existing in remote places and discloses an asynchronous data duplication technique which does not exert an influence on the write response by storing primary data sent from the host to a primary data control unit, after that, sending secondary data as a copy of the primary data from the primary data control unit to a secondary data control unit as a copy destination in a position which is geometrically and physically remote from the primary data control unit.
According to the International application published under the patent cooperation treaty WO 94/25919, the primary data control unit is provided with a primary data information system, the secondary data control unit is provided with a secondary data information system, and an asynchronous copying process from the primary data control unit to the secondary data control unit is performed. It is not, however, described that the primary and secondary data information systems assure consistency of data in the asynchronous data duplication.
It is therefore an object of the invention to provide a function of assuring not only consistency of data but also the limit of the assurance from various points of view, and to assure significance of the consistency of data by assuring and maintaining the data consistency within the range expected by the user.
In order to solve the problems, according to the invention, there is provided an information storage system comprising: one or more master control units each having a memory and one or more storing units, for receiving a data write request from an information processor and storing the write data; and one or more remote control units each having a memory and one or more storing units, to which the write data is transferred from the master control unit and which stores the write data, wherein any of the control units includes a control module for comparing a time difference between the latest time and the oldest time among added write time in a group of write data which has not been transferred but should be transferred to the remote control unit with a preset allowance time.
When the time difference exceeds the allowance time, the control module may regulate a write request from the information processor, or regulate or interrupt writing of data to the remote control unit as a destination of write data.